In this technical field, Automatic Repeat reQuest (ARQ) method is utilized for improved communication reliability. In the ARQ method, received packets are subjected to error detection, and the transmitter side is requested to retransmit packets including unacceptably significant errors. The error detection may be conducted in accordance with any existing technique such as CRC (Cyclic Redundancy Check) scheme. In response to receipt of the request, the transmitter side retransmits the requested packets. As a result, it is possible to partially prevent an application in the receiver side from running without any packet.
FIG. 1 shows an exemplary operation of the ARQ method. For example, Internet Protocol (IP) based packets A, B, C, . . . are received from an upper apparatus and stored in a reception buffer in a base station in the order (S1). The received IP packet is divided into fixed length blocks for transmission in a lower layer (S2). Then, radio transmission of a predefined number of blocks is sequentially carried out via physical channels, and the blocks are received at a mobile station (S3). In the illustrated example, two blocks are transmitted in each transmission time interval (TTI). The mobile station conducts error detection on the received packets. The received packets are stored in a RLC (reception) buffer in the mobile station (S4). In this case, retransmission requests for error detected packets are transmitted to the base station. In the illustrated example, errors are detected for the 11th and 12th blocks as illustrated in “NG”, and the mobile station requests the base station to retransmit the blocks. The mobile station forwards error free packets to layers above the RLC such as an application layer (S5). A packet including the detected error, which corresponds to packet C in the illustration, is discarded, and then the mobile station waits for an appropriate packet and after completion of the retransmission procedure, forwards the retransmitted packet to an upper layer.
For further improvement in the communications reliability, a Hybrid ARQ (HARQ) scheme resulting from the error correction scheme being combined with the ARQ scheme is proposed. This may be also employed in the HSDPA (High Speed Downlink Packet Access) scheme.
FIG. 2 is a schematic block diagram illustrating a base station and a mobile station in accordance with the HARQ scheme, and FIG. 3 shows an exemplary operation of the HARQ scheme. IP packets A, B, C, . . . are received from an upper apparatus and stored in an RLC/ARQ or reception buffer (FIG. 2) in a base station in order (S1 in FIG. 3). The received IP packets are divided into fixed length blocks in the RLC buffer in FIG. 2 (S2). Then, one or more fixed length blocks are bundled and transmitted in an HARQ-PDU processing unit depending on the radio environment. The individual blocks are identified with the sequence number (SN=1, 2, . . . ). The IP packets are radio transmitted via physical channels in unit of a predefined number of blocks and are received in a mobile station (S3). Packet data units including one or more blocks are assigned the respective numbers (TSN=1, 2, . . . ), and processes for transmitting information items specified by the numbers are assigned process numbers (P1, P2, . . . ). The mobile station performs error detection on the received packets, and if an error is detected as illustrated in “NG”, the mobile station requests the base station to retransmit the associated packet. The received packets are stored in a reordering buffer illustrated in FIG. 2 (S4). The reordering buffer holds packets not being in need of retransmission until missing packets are received. In the illustrated example, an error is detected for a packet data unit corresponding to TSN=2 (whose process number is P2), and the mobile station requests the base station to retransmit it. In response, the base station conducts retransmission. In the illustrated example, the retransmitted packets are received without any error and stored in the reordering buffer. At this stage, the retransmission requests are carried out in a MAC sublayer. The stored packets are reordered in the reordering buffer in order of the TSN number. The properly reordered packets are decomposed into packet data units (RLC-PDUs) in the RLC layer and then stored in the RLC buffer (S5). It is determined whether the packets stored in the RLC buffer are also continuous in terms of the sequence number SN. The determination is made because in an exemplary case of an error still being detected even after the maximum retransmission times are reached in the HARQ, a missing packet may still be detected in a packet in the RLC buffer. In such a case, retransmission in the RLC layer is requested by the mobile station to the base station. At this stage, the retransmission request operations are carried out in the RLC layer (also referred to as an ARQ layer). The mobile station forwards error free packets to an upper layer such as an application layer from the RLC. For packets including error detected blocks, the mobile station waits for an appropriate packet and after completion of the retransmission procedure, forwards the retransmitted packets to the upper layer (S6). The above-mentioned conventional ARQ and HARQ methods are described in non-patent document 1 publicly disclosed at the filing date of the base priority application of the present application.
Non-patent document 1: 3GPP TS 25.301 6.4.0, Internet <URL: http://www.3gpp.org>